This invention relates to a filter circuit, and is more particularly suitably applied to a filter circuit of a capacity grounded type.
In conventional filter circuits which are constituted by combining transistors, capacitances and resistances, there are filter circuits (that is, multiple input-one output type filters) which are desirably constructed by inputting input signals to one of the input terminals.
As illustrated in FIG. 1, such a filter circuit 1 is constituted by two stage mutual conductance variable type operational amplifiers 2 and 3 and a buffer 4. Input signals V.sub.INL are inputted to a non-inverting input terminal of the operational amplifier 2, input signals V.sub.INB and V.sub.INH are inputted to non-inverting input terminal of the operational amplifier 3 and the input terminal of the buffer 4 through condensers C2 and C1, respectively, and then are outputted as output signals V.sub.0 from the buffer 4.
The output signals V.sub.0 are fed back to the inverting input terminals of the operational amplifiers 2 and 3.
This filter circuit 1 may be represented by an equivalent circuit as shown in FIG. 2 by indicating the operational amplifier 5 and the output side condenser C3 (FIG. 3A) by an integrator 5A, an input side subtracter 5B, and an output side adder 5C (FIG. 3B) as illustrated in FIG. 3.
Here, the transmission function of the output signals V.sub.0 can be expressed by the following equation, using input signals V.sub.INL, V.sub.INB and V.sub.INH : ##EQU1##
Here, using angular frequencies .omega..sub.1 and .omega..sub.2 of the first and second operational amplifiers 2 and 3, .omega..sub.0 and Q may be given by the following equation: EQU .omega..sub.0.sup.2 =.omega..sub.1 .multidot..omega..sub.2 (2) ##EQU2##
Using mutual conductance g.sub.1 and g.sub.2 of the first and second operational amplifiers 2 and 3, angular frequencies .omega..sub.1 and .omega..sub.2 are given by the following equations: EQU .omega..sub.1 =g.sub.1 /C.sub.1 (4) EQU .omega..sub.2 =g.sub.2 /C.sub.2 (5)
More specifically, the filter circuit 1 constitutes a low-pass filter as shown in the following equation: ##EQU3## in the case where only input signals V.sub.INL are inputted (hereinafter referred to as capacity ground). This occurs when in Eq. (1), V.sub.INB =V.sub.INH =0. On the other hand, the filter circuit 1 may constitute a high-pass filter as shown in the following equation: ##EQU4## This occurs when input signals are inputted through condensers (C2 and C1) (hereinafter referred as floating input). This may occur, for example, only when input signals V.sub.INH are inputted, that is, when in Eq. (1), V.sub.INL =V.sub.INB =0.
In such a filter circuit 1, only the low-pass filter can be however constituted by the capacity ground input since it is not possible to apply is to a case where high-pass filter is designed using a capacity ground input filter.
Although it might be possib1e to constitute a filter circuit using one input multiple output type filter circuit (FIG. 4) and multiple input-multiple output type filter circuit (FIGS. 5 and 6), but in either case, it is not possible to constitute a high-pass filter with a simple configuration according to the capacity ground input.
More specifically, in the case of one input-multiple output circuit 10, output signals V.sub.01, which are outputted through the first and second integrators 11A and 12A, and output signals V.sub.03, which are outputted only through-the first integrator 11A, constitute a low-pass filter and a band-pass filter in transmittance, respectively. Output signals V.sub.02 which are outputted through the first subtracter 11B can only a high-pass filter including a first order term in the numerator of Eq. (7).
In the cases of a feedback type secondary filter 13 (FIG. 5) and a feedforward type secondary filter 16 (FIG. 6) which constitute multiple input-multiple output type filter circuits, it is not possible for output signals V.sub.01 and V.sub.02 which are outputted from the first and second output terminals on the basis of input signals V.sub.INA to constitute high-pass filters as shown in FIG. 7.